CI: cargo-update from the right directory

Without this change, the patch.crates-io does not take, and we are
building with an old version; this primarily causes issues during
version updates when the version found through patch.crates-io at
runtime is recognized as the new one, but the lock file says to use the
released version, and in the end cargo falls back to not using the local
version.

.github/workflows/test.yml

@@ -105,7 +105,10 @@ jobs:
       # and all relevant crates once, and after that, just make some notes in Cargo.lock
       run: |
         set -x
-        for MANIF in $(find RIOT -name Cargo.toml)
+        # It is important to cd in early, for otherwise the patch.crates-io
+        # will not catch on during the update
+        cd RIOT
+        for MANIF in $(find -name Cargo.toml)
         do
             echo "::group::Updating ${MANIF}"
             cargo update -p riot-sys -p riot-wrappers --aggressive --manifest-path $MANIF